The present invention relates to improvements in blast-furnace equipment and more particularly to improvements in the supports for a blast-furnace hot-blast main.
The hot-blast main is used to supply a blast furnace with air heated in air heaters.
The hot-blast main consists of a trunk section rigidly and airtightly connected to the air heaters through hot-blast connecting pipes, and a bustle pipe section joined to the blast furnace by means of tuyere connections. The hot-blast main is manufactured of large diameter (1700 to 3500 mm) welded steel tubing lined inside with refractory materials.
The hot-blast main usually supplies the blast furnace with air heated up to 1000.degree. C at up to 3 atm pressure.
At present, the average hot-blast temperature in modern blast-furnace installations is up to 1200.degree. C with anticipated increases up to 1400.degree. C at blast pressures up to 5 atm. These increased temperatures help to reduce coke consumption and, consequently, the cost of the cast iron produced.
In the majority of the known designs of hot-blast mains the trunk section with the connecting pipes forms in a horizontal plane a rigid three- or four-span (depending on the number of air heaters) frame, with the air heaters used as supports. In a vertical plane, this structure is, as a rule, suspended at several points by means of flexible rods from carrying beams whose ends are resting on cross-bars. The crossbars in turn are resting on self-contained supports (columns).
In the course of use, especially at high hot-blast temperatures and pressures, a considerable number of defects (over-heats, distortions, cracks) occur in the metal housings of the hot-blast main and the air heaters at the points where the connecting pipes join the trunk section of the main and the heaters. Elimination of overheats, cracks, etc. is difficult (due to the noxious and gas-polluted environment and high temperatures) and involves a considerable loss it necessitates product because of the disrupting the operation of the blast furnace. To avoid these disruptions it is necessary to prevent the major causes of overheats, distortions and cracks in the structures.
The rigidity of the hot-blast main structures in a horizontal plane precludes free displacement of its conjugated sections during heating. Stresses at the points of conjugation of these sections between each other and with the air heaters, as calculations and tests have revealed, exceed the yield limits of the grades of steel used to manufacture the housings of the structures.
The design of the supports which take the vertical loads induced by the mass of the hot-blast main proper is inherently substantially disadvantageous, because, in the course of operation, the load can be fully transmitted to the connecting pipes to cause their bending.
This results from the vertical displacement of the air heaters to which the connecting pipes are rigidly secured, as has been noted above.
The vertical displacement of the air heaters, regularly variable in the course of operation, is caused by the increased temperature (up to 100.degree. C) of their housings and the cyclic operation of the air-heaters (blasting-heating).
Vertical displacement of the carrying beams from which the trunk section of the hot-blast main is suspended depends on the load transmitted to their supports, and also on daily and seasonal variations in the ambient air temperature.
The relative difference between the vertical displacements of the carrying beams and the hot-blast connecting pipes joining the air heaters is considerable (30-60 mm), depending on the difference in the level of installing the connecting pipes and the air heater base and also on the method of fixing the air heaters to the foundation.
The mass of the trunk section itself amounts to 10 tons per linear meter. Therefore the additional stresses in the housings due to the bending of the connecting pipes (100-200 ton load at 6-10 m arm) may exceed the yield limit of the grades of steel used to manufacture the housings, which leads to the recurrence of cracks and overheats, impairs the replacement of hot-blast valves fitted to the connecting pipes, and reduces the service life of the hot-blast main.
A supporting device for a hot-blast main is known which is intended for the elimination of the aforesaid disadvantages (Inventor's Certificate No. 357,334, USSR). The problem is solved by means of hydraulic cylinders which support the ends of the carrying beams; the cylinders are controlled through a follow-up system which senses the vertical displacements of the connecting pipes.
There are other structural designs of hot-blast mains which are known for eliminating the above-mentioned disadvantages. At blast temperatures of 1200.degree. - 1400.degree. C, these mains include supports in the trunk section located along the air heaters (in plan). The section is divided into separate lengths interconnected through temperature compensators in the form of corrugated steel shells with gaps in the refractory lining within their zones to compensate for displacements due to temperature variations. Each section is joined to the air heater by means of the connecting pipe, whereas each connecting pipe is additionally fitted with one or two temperature compensators similar to those described above. The hot-blast mains of this design are presently employed by producers in the Federal Republic of Germany, Japan, and France.
It should be noted, however, that all the above cited designs are complicated and costly. Thus, the application of hydraulic cylinders with follow-up systems involves the necessity of their constant attendance and adjustment.
The installation of temperature compensators results in longer connecting pipes, necessitates the use of additional supports for the compensators, and requires special holders to take up unbalanced forces arising due to the incresed internal blast pressure. All this adds much to the complexity and cost of the hot-blast main structures.